The invention relates to heat management in a chemical process.
There are a number of chemical processes that involve hydrogen at elevated pressure, including hydroformylation, hydrocarbonylation, hydroaminomethylation, and hydrocyanation of olefins. Each of these must deal with the presence of feed stream impurities, which can include N2, Ar, CO2, methane, higher alkanes, water, and the like. These impurities typically are purged from the process in a vent stream.
For example, the hydroformylation of olefins, such as propylene, is carried out industrially in a continuous process in that the olefin, carbon monoxide and hydrogen are reacted in a hydroformylation reactor in the presence of a hydroformylation catalyst. The output from the reactor comprises the hydroformylation products (aldehydes, alcohols) and generally significant amounts of unreacted olefin that must be separated off and recirculated to the hydroformylation reactor. When olefins are subjected to hydroformylation, i.e., an Oxo reaction at 50° C. to over 200° C. and pressures as low as 3 bar and as high as over 200 bar, gas mixtures are obtained in addition to the liquid reaction products such as aldehydes and alcohols. These gas mixtures consist of the unconverted reactants and also of alkanes from the olefin feed as an impurity and from side reaction hydrogenation of the olefin. Conventional olefin hydroformylation reaction schemes must allow for venting of inerts such as N2, Ar, CH4, alkanes, CO2 and the like. In the case of propylene hydroformylation, these gas mixtures have been burned as off-gases, since the recovery of olefin, CO and H2 from the off-gases is not cost effective. These gas mixtures together with the i-butyraldehyde obtained as an isomeric by-product, have been partially oxidized to produce the syngas, (i.e., carbon monoxide and hydrogen), olefin, and hydrogen required for the hydroformylation. However, after the enormous increase in the price of propylene, the conversion of valuable propylene to syngas is no longer economical.
In general, it would be desirable to recirculate the unreacted olefin back to the reactors to get maximum conversion. However, this recirculation also recycles the inert propane that is present as an impurity in the feed propylene, or formed by side reactions in the hydroformylation reactor. To prevent the propane concentration in the hydroformylation reactor from rising continually and reaching values at that the hydroformylation reaction ceases, a sub-stream of the recirculated propylene-containing stream must be continually bled off from the process in order to remove the inerts and propane.
However, unreacted propylene also is removed from the system by the bleed stream. To keep propylene losses small, a propylene feed of high purity is generally used. Thus, hydroformylation is usually carried out using a propylene feed having a purity of about 99.5%, with the remainder consisting essentially of propane. This grade of propylene is referred to as “polymer grade propylene.” Such high purity propylene is sold at significantly higher prices than propylene of lower purity. For example, “chemical grade propylene” containing from about 3 to 7% by weight of propane is significantly cheaper than polymer grade propylene.
For the reasons mentioned above, a propylene feed having a relatively high proportion of propane cannot be used in an industrial hydroformylation process without taking appropriate measures.
Reactor vent streams contain valuable olefin, syngas, and product, that is lost to the fuel header or flare. The prior art has many ways of recovering reactants. Examples of secondary reactors on these vents to maximize conversion are known; see, e.g., GB 1,387,657, U.S. Pat. No. 4,593,127, U.S. Pat. No. 5,367,106, U.S. Pat. No. 5,426,238, U.S. Pat. No. 7,405,329, WO 2010/081526 and WO 2010/115509. Nevertheless, a vent purge of inert gases and alkanes is still present.
Significant prior art focuses on recovery of the contained olefin, such as propylene, in these streams, but little attention has been focused on product loss minimization. For example, U.S. Pat. No. 4,210,426 employs an extensive absorption/desorption scheme with up to 3 columns to extract the olefin from the vent stream. This capital intensive process is complex. There is no mention of recovering any contained aldehyde product.
Separation processes that require a foreign substance as a recovery agent are known, using agents such as diethylpropionamide, methanol, aromatic compounds, acetonitrile, dimethoxytetraethylene glycol, hydrocarbons and aldehyde heavies. However, these processes have a considerable disadvantage, in that the gases recovered from the off-gases require careful purification to remove the particular recovery agent before the gases are re-employed in the hydroformylation process. It is not clear how an aldehyde product could be recovered from these systems without expensive refining.
U.S. Pat. No. 5,001,274, U.S. Pat. No. 5,675,041, U.S. Pat. No. 6,822,122, U.S. Pat. No. 6,100,432, and JP 4122528 discuss using vent scrubbers and vent distillation schemes to recover unreacted olefins. Again, these complex schemes are focused on recovering the olefin and only U.S. Pat. No. 5,001,274 has any discussion of recovering any contained aldehyde or alcohol product in the vent stream. However, U.S. Pat. No. 5,001,274 also captures inerts such as alkanes, which may interfere with the inert purge.
Pressure-swing absorption (PSA) and related technologies to separate propylene and recycle it to the hydroformylation zone are taught by U.S. Pat. No. 5,463,137 and U.S. Pat. No. 5,483,201. Membrane technologies are also applicable, as taught in U.S. Pat. No. 6,414,202. CN 101774912 A1 teaches using PSA to recover syngas to be recycled back to the hydroformylation zone. All of these are capital intensive, and the complex mixture of polar aldehydes and their interaction with the other components tend to interfere with these technologies, especially for long term use. The alternating adsorption and desorption cycles require periodic pressure and/or temperature changes. The equipment required for PSA are complicated and susceptible to malfunctions.
Refrigeration to cool the vent stream to condense the product is an available option, but refrigeration is expensive and high maintenance; see, e.g., U.S. Pat. No. 4,287,369 Ammonia, a common refrigerant, is reactive with aldehydes. Thus, any leaks could have highly undesirable consequences. Conventional cooling water is typically around 40° C. and a significant amount of aldehyde can still be present due to a significant vapor pressure at that temperature. The use of propane as a coolant is known. For example, in U.S. Pat. No. 4,210,426, purified propane decompression is used to cool the reflux condenser in an aldehyde-propane distillation column. In this case, the condenser is used to recycle the absorbent fluid. The propane stream is quite pure, as hydrogen, inert gases, etc. were removed at an earlier stage. This process requires two columns to purify the propane before it is used as a refrigerant.
U.S. Pat. No. 6,864,391 discloses a process in which the vent stream is oxidized to convert the contained olefin into other more readily isolated products, such as the corresponding acrylic acid, but only after extensive refining to remove traces of aldehyde from the stream.
All these processes are unsuitable for the recovery of aldehyde product from the off-gases from the hydroformylation of propylene, since they are much too expensive.
It would be desirable to have a simple and economical process for the recovery of products, such as aldehydes, from reactor vent streams.